Presses – With additional treatment of material – Heating – cooling – or drying
Patent
1996-02-13
1999-04-27
Gerrity, Stephen F.
Presses
With additional treatment of material
Heating, cooling, or drying
100153, 100160, 100162B, 162206, 1623585, D21G 100, B30B 304
Patent
active
058968130
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention concerns a gradient calender, comprising two calender rolls which form a calendering nip between them, of which calender rolls at least one roll is a hard-faced calender roll which can be heated to a high temperature, the paper web or equivalent being passed through the calendering nip for hot calendering of the web.
BACKGROUND OF THE INVENTION
By means of calendering of paper, attempts are made to apply an improving effect to smoothness, glaze and surface properties of paper. The calendering temperature has a significant effect on the properties of paper so that, for example, if it is desirable to keep the smoothness that has been achieved invariable, by raising the temperature it is possible to lower the linear load that is used. On the other hand, if it is desirable to keep the linear load unchanged, the smoothness of the paper can be improved by raising the calendering temperature. This is why, in paper machines, paper is hot-calendered, for example, by means of a what is called gradient calender. Thus, today a gradient calender comprises two heatable rolls that form a nip with one another, the paper web being passed through the calendering nip between said rolls. It is a problem of the present solutions that at least one of the heatable rolls that form the calendering nip must be a variable-crown roll in order that the desired uniform linear load could be obtained in the calendering nip. However, making a variable-crown roll into a hot roll is highly expensive, and especially when surface temperatures of an order of 200.degree. C. and higher are required from calender rolls, the limits of the present-day technology constitute an obstacle. In the light of the prior-art knowledge, the service lives of hydraulic fluids and of hydraulic components in general become decisively shorter when the temperature of the fluid exceeds 200.degree. C. At the same time, the cost of operation of the system becomes unduly high. These problems are manifested in particular in connection with a heatable variable-crown roll. It is a second factor which increases the costs significantly that in the prior-art calenders the diameters of the calender rolls must be quite large in order that the construction could be made robust enough.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the present invention is to provide a gradient calender of a novel type which provides a significant improvement over the prior art and by whose means most of the drawbacks related to the prior-art solutions are avoided. In view of achieving this, the gradient calender in accordance with the invention is mainly characterized in that at least one of the calender rolls is arranged to be loaded towards the calendering nip by means of a variable-crown support roll so that the auxiliary nip between said support roll and the corresponding calender roll forms a common nip plane with the calendering nip, deflection of the calendering nip and of the calender rolls, respectively, being substantially prevented by the effect of the load produced by means of the hydraulic loading means in the support roll/rolls.
By means of the invention, a number of remarkable advantages are obtained over the prior art, of which advantages some of the most important ones will be described in the following. Even if the construction of the gradient calender in accordance with the present invention is more complicated than in the prior art, the servicing and maintenance of the system is, nevertheless, considerably simpler and easier. The cost of operation of a gradient calender in accordance with the present invention is substantially lower than in the prior art. The variable-crown rolls that are used in the calender in accordance with the present invention are at a normal temperature, so that the problems of components related to the prior art are eliminated. Nor does the solution in accordance with the invention involve sealing problems similar to those in the prior-art solutions, because in the present invention none of the rolls th
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"Thermal Bonding Utilizing Calenders", D. Steve Gunter, Nonwovens Bonding, TAPPI Journal, Jun., 1994, pp. 216-224.
Lehto Ari
Nikulainen Osmo
Gerrity Stephen F.
Valmet Corporation
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